[][src]Crate better_any

Better Any

Rust RFC for non_static_type_id feature has been reverted. Which means in foreseeable future there will be no built-in way in rust to get type id for non-static type let alone safely use it to downcast to a particular type.

This crate provides tools to do these things safely for types with single lifetime. Although looks like it is technically possible to extend this approach for multiple lifetimes, consistent api and derive macro would be much harder to create and use because of the necessity to properly handle lifetime relations. Feel free to create an issue if you have actual use case where you need this functionality for multiple lifetimes.

Also it has better downcasting that allows you do downcast not just from dyn Tid (like dyn Any) but from any trait object that implements Tid. So there is no more need to extend your traits with fn to_any(&self)-> &dyn Any

MSRV: 1.41.0-stable


Basically in places where before you have used dyn Any you can use dyn Tid<'a>

  • If your type is generic you should derive Tid implementation for it with Tid derive macro. Then to retrieve back concrete type <dyn Tid>::downcast_* methods should be used.
  • If your type is not generic/implements Any you can create dyn Any and convert it to dyn Tid. Then to retrieve back concrete type <dyn Tid>::downcast_any_* methods should be used
  • If your type is not generic and local to your crate you also can derive Tid but then you need to be careful to use methods that corresponds to the way you create dyn Tid for that particular type. Otherwise downcasting will return None.

If all your types can implement Tid to avoid confusion recommended way is to use first option even if some types implement Any. If there are some types that implement Any and can't implement Tid (i.e. types from other library), recommended way is to use second option for all types that implement Any to reduce confusion to minimum.

Interoperability with Any

Unfortunately you can't just use Tid everywhere because currently it is impossible to implement Tid for T:Any since it would conflict with any other possible Tid implementation. To overcome this limitation there is a From impl to go from dyn Any to dyn Tid But Any and Tid deliberately return different type ids because otherwise Type<'a> and Type<'static> would be indistinguishable and it would allow to go from Type<'static> to Type<'a> via dyn Tid which is obviously unsound for invariant and contravariant structs.

Although if you are using dyn Trait where Trait:Tid all of this wouldn't work, and you are left with Tid only.


It is safe because created trait object preserve lifetime information, thus allowing us to safely downcast with proper lifetime. Otherwise internally it is plain old Any.



Simple version of derive macro to not pull all proc macro dependencies in simple cases when all structs are not generic



This trait indicates that this type can be converted to trait object with typeid while preserving lifetime information. Extends Any functionality for types with single lifetime


This trait indicates that you can substitute this type as a type parameter to another type so that resulting type could implement Tid.


Extension trait that contains actual downcasting methods.



Returns type id of T

Attribute Macros


Attribute macro that makes your implementation of TidAble safe Use it when you can't use derive e.g. for trait object.

Derive Macros


Derive macro to implement traits from this crate